I live in a low rain area, but all of my neighbors that have built these systems have used 40k gallon tanks. They stay full most of the year with little problem.
I asked AI "how much annual rainfall do you need to keep a 40k water collection unit full"
To determine the exact annual rainfall needed, you must multiply your roof catchment area (in square feet) by a conversion factor of 0.623. For example, a 2,000 square-foot roof yields about 1,246 gallons for every inch of rain, requiring about 32 inches of annual rain to collect 40,000 gallons.
1,500 sq foot roof - 43"
2,500 sq foot roof - 26"
Phoenix averages about 7.5" per year. People use more water than the climate provides in that area. Not all areas are the same, so what works in Bermuda or Texas, will not work in Phoenix.